1. Field of the Invention
The present invention relates to a waveguide type optical modulator used in optical communication, in particular, an optical modulator having a multistage configuration in which a plurality of optical modulators formed on an identical substrate are connected to each other.
2. Description of the Related Art
For example, an optical waveguide device using electrooptic crystal of lithium niobate (LiNbO3), lithium tantalate (LiTaO2), or the like is formed as follows. A structure obtained by forming a metal film on a part of a crystal substrate is thermally diffused, or patterned and then subjected to proton exchange in a benzoic acid to form an optical waveguide. Thereafter, electrodes are arranged near the optical waveguide. As one of the optical waveguide devices using electrooptic crystal, for example, an optical modulator or the like using a Mach-Zehnder (MZ) type optical waveguide is well known.
A general MZ type optical modulator includes: an optical waveguide consisting of an MZ interferometer constituting an input waveguide, a branching unit, a pair of branch waveguides, a coupler, and an output waveguide; and coplanar electrodes obtained by arranging a signal electrode and a ground electrode on the pair of branch waveguides. More specifically, for example, since a change in refraction index obtained by a z-direction electric field is used when a z-cut substrate is used, the signal electrode and the ground electrode are arranged immediately above the branch waveguides. Although the signal electrode and the ground electrode are patterned on the branch waveguides, respectively, in order to prevent light propagated in the branch waveguides from being absorbed by the signal electrode and the ground electrode, a dielectric layer (buffer layer) is arranged between the substrate and the signal electrode and the ground electrode. As the buffer layer, for example, a silicon oxide (SiO2) film or the like having a thickness of 0.2 to 2 μm is used.
When the optical modulator is driven at a high speed, an output terminal of the signal electrode is grounded through a resistor to obtain a traveling-wave type electrode, and a high-frequency electric signal such as a microwave is applied from an input terminal of the signal electrode. At this time, refraction indexes of the branch waveguides are changed by an electric field generated between the signal electrode and the ground electrode to change a phase difference of lights propagating in the branch waveguides, whereby signal light modulated in intensity is outputted from the output waveguide. Furthermore, with respect to the optical modulator driven at a high speed, the following fact is known. That is, a sectional shape of the signal electrode is changed to control an effective refraction index, and propagation speeds of the light and the electric signal are matched with each other, so that wide-band optical response characteristics are achieved.
Furthermore, the following optical modulator is also known. That is, two MZ type optical modulators are connected to each other in tandem, an electric signal corresponding to a clock is applied to a signal electrode of one MZ type optical modulator, and an electric signal corresponding to NRZ (Non-Return to Zero) data is applied to a signal electrode of the other MZ type optical modulator, so that an optical signal of an RZ (Return to Zero) modulation method can be generated. In the optical modulator of the RZ modulation method, since two MZ type optical modulators are arranged in series with each other in a propagating direction, the length of a chip is twice that of an optical modulator of an NRZ modulation method using one MZ type optical modulator. Furthermore, although a drive voltage decreases when an interaction length increases, since an interaction length in an optical modulator of the RZ modulation method is limited by a chip size, there is a problem in that the drive voltage cannot be easily reduced.
Therefore, the present applicant proposes the following configuration. That is, two MZ type optical modulators are arranged in parallel, and the two MZ type modulators are connected to each other by using a curved folded waveguide (for example, see WO 2004/068221). More specifically, as shown in FIG. 11, two MZ type optical waveguide units 120A and 120B are arranged on in parallel to each other on an identical substrate (chip) 110, one terminals of the MZ type optical waveguide units 120A and 120B are located on the same end face of the substrate 110, and the other terminals are connected to each other through a curved folded waveguide 121. Coplanar electrodes are patterned in association with the MZ type optical waveguide units 120A and 120B. In this case, to one terminal of the MZ type optical waveguide unit 120A located at the lower left of a signal electrode 131A in FIG. 11, an electric signal CLK having a clock waveform indicated by the first stage in FIG. 12 is applied. To one terminal of the MZ type optical waveguide unit 120B located at the lower right of a signal electrode 131B in FIG. 11, an electric signal DATA having NRZ data as indicated by the second stage in FIG. 12 is applied. In this manner, incident light Lin is propagated through the MZ type optical waveguide unit 120A on the input side to obtain a light signal La having a waveform as shown in the third stage in FIG. 12. Furthermore, the light signal La is propagated through the curved folded waveguide 121 and the MZ type optical waveguide unit 120B on the output side to obtain a RZ-modulated light signal Lout having a waveform as indicated by the fourth stage in FIG. 12.
In relation to the configuration using the curved folded waveguide as shown in FIG. 11, a configuration in which a curved folded waveguide is applied to a central portion of a pair of branch waveguides in one MZ type optical modulator is also proposed (for example, see Japanese Patent Application Laid-Open No. 2005-221874).
In the optical modulator having the conventional configuration as shown in FIG. 11, the curved folded waveguide 121 has a loss which increases when the radius of the curved folded waveguide 121 decreases. For this reason, the curved folded waveguide 121 requires a radius of 2 mm or more in general. Two input connectors to supply electric signals to the signal electrodes 131A and 131B of the two MZ type optical modulators must be arranged near one side surface (side surface on the lower side in FIG. 11) of the substrate 110 to make it easy to package the input connector. For this reason, a feeder part which guides an electric signal to a signal electrode (signal electrode 131B in FIG. 11) of the MZ type optical modulator which is farther from the input connector becomes long to disadvantageously increase a loss of the electric signal. Furthermore, a chip length decreases because the two MZ type optical modulators are arranged in parallel to each other. However, a chip width is not easily made smaller than a predetermined width because the chip width is restricted by the radius or the like of the curved folded waveguide 121. For this reason, the number of chips obtained from one wafer is disadvantageously limited to a specific number.